I've had a slow coolant leak on the GTI for at least a couple years now. I add maybe 1/2 cup of coolant every few months to top it up. Never saw a puddle under the car or smelled a leak, but I didn't look too carefully.
Last spring I figured I should probably do something about it, so took a closer look. Found a tiny pool of coolant on an underbody tray, and there seemed to be a bit of seepage around the secondary pump. The secondary pump is cheap and easy to replace, so I did that and thought the problem was fixed...
A few months later, coolant is low again.
Now I'm pretty sure it's seepage from the main pump which trickles down the hoses and front of the engine. Water pump is a hidden under the intake manifold and is a PITA to replace, so I'm just ignoring it again. Okay, not ignoring, just topping up the coolant every few months. Again, only 1/2 cup each time, so not a big deal. Doesn't seem to be getting any worse.